A resonator optical gyroscope is a rotation rate sensing device that includes a resonant cavity. The resonant cavity supports light waves propagating in opposite directions (without loss of generality, they are referred in the following to clockwise (CW) and counter-clockwise (CCW) directions, respectively). When there is a non-zero rotation rate around the normal axis of the resonator, the effective optical round-trip path length for the CW and CCW lightwaves is different, leading to a resonant frequency difference between them. By measuring this resonant frequency difference, rotation rate can be determined.
Resonator fiber optic gyroscope (RFOG) is a special kind of resonator gyroscope that uses optical fibers in the resonator. Optical fiber increases the gyro signal-to-noise (S/N) sensitivity without significantly increasing the size of the sensing loop. For measuring the resonant frequency difference, monochromatic light waves are typically sinusoidally phase/frequency modulated and coupled into the RFOG resonator in the CW and CCW directions. Fractions of light circulating inside the resonator are coupled out of the resonator and converted to electronic signals at photodetectors. The electrical signals are demodulated at the corresponding modulation frequencies and used to servo the input light frequencies to the resonance frequencies of the CW and CCW cavity.
Along the optical path before and after the RFOG's phase modulator, there can be polarization cross-couplings points due to imperfect fiber splices or polarization axis mismatch between the modulator waveguide and its pigtail fibers (not shown). In this case, the phase modulation behaves like a Mach-Zehnder interferometer with its two optical arms formed by the two orthogonal polarization paths of the modulator. Most of the optical power propagates in the optical path whose polarization state is aligned with the pass-axis of the modulator. A small amount of (cross-coupled) optical power propagates in the optical path whose polarization state is orthogonal to the pass-axis of the modulator. At a cross-coupling point after the phase modulator, the interferences between the light waves propagating along the two orthogonal polarization axes of the modulator causes intensity modulation at the phase modulation frequency, leading to erroneous demodulation of the rate signals. Erroneous demodulation generates bias errors of the gyro.
Ways to reduce this modulator induced intensity modulation are through selection of phase modulators with high polarization extinction ratio (PER) and reduction of polarization cross-couplings in the gyro optical path. These require high performance modulators and increase production complexity and cost. An unobvious but clever way to solve the problem is to proper select the phase modulation amplitude (to be detailed in this invention). Care must be taken in this method because change of phase modulation amplitude affects the gyros S/N sensitivity (which determines how fine a rotation rate can be measured). It is necessary to find a way that simultaneously reduces the bias error and maximizes the gyro S/N sensitivity.